In your wrapper.conf file you should have a classpath delcaration similar to wrapper.java.classpath.3=/opt/SFS_PRO_1.6.6/Server/javaExtensions/. If not, then place that line in your file (noting the classpath.# format and the full path to your extensions folder).

The way you have it now is fine, but if you want to further separate your extension code from the smartfox server resources, then you can specify any path you would like in your wrapper. The important thing is that it is accessible from your classpath.